1. Field of the Invention
The present invention relates generally to signal filtering, and more particularly to tunable RF filters.
2. Description of the Related Art
Tunable RF filters are useful for software-defined radio applications. Very wideband frequency coverage may be desirable in such applications, often from tens of MHz to several GHz. The tuning requirements may therefore differ greatly from those which are conventionally pursued for civilian telecommunication systems.
Tunable and band-select filters are commonly designed by integrating solid-state/MEMS varactors or switches (Brown and Rebeiz, 2000; Entesari and Rebeiz, 2005; Abbaspour-Tamijani et al., 2003; Siegel et al., 2005; Park et al., 2006; Sanchez-Renedo et al., 2005) within the structure of conventional lumped or distributed bandpass filters. Most commonly, tuning response is attained by changing the capacitors or inductors in the resonators of a bandpass filter. One limitation that arises in most of these structures is caused by the spurious pass-bands that occur due to the higher order resonances. Spurious pass-bands have almost always been treated as undesirable artifacts in band-select filters that are intended to deliver only one frequency band at each mode of operation.